In the manufacture of light alloys, characteristic of which is a high chemical activity in molten state, much consideration is given to the removal of nonmetallic impurities from the melt. The requirements upon the purity of metals and alloys in terms of hydrogen content, especially of solid nonmetallic oxide inclusions are growing ever more stringent. For example, no oxide inclusions larger than 10 .mu.m are admissible in a number of aluminium alloy items such as foil for capacitors.
On the other hand making ingots from high-strength light alloys of medium and large size of section (e.g. up to 120 cm in diameter or with a cross-sectional size of up to 40.times.120 cm and greater) is characterized by a coarse-grained feathery structure, increased hydrogen content and porosity even in casting a vacuum-treated metal.
The way in which the structure of large ingots is formed and the resulting porosity lower the plasticity of the ingots and increase the tendency of the ingots to crack on casting, this restricting the dimensions of sound ingots which can be cast and decreasing alloy plasticity in subsequent press-working.
These aspects of the light alloy continuous casting underlie a wide industrial use of ultrasonic treatment of melt for effective purification of metal and refinment of the cast structure.
There is known a method for continuous casting of light-alloy ingots (cf. USSR Inventor's Certificate No. 353,790, Cl. B 60 B, 1972), comprising pouring a melt, acting upon the melt with at least one radiator for purifying the melt and refining the structure of a solidifying ingot and simultaneously withdrawing the ingot.
This method is put into effect at high rates of casting (for example, 30 cm/min), and the melt is treated with ultrasound for a short time, so that ingots of small cross-section may be cast. In application to casting ingots of medium and large cross-sections, this method is of a limited usefulness as the casting rate drops substantially (1 to 2 cm/min) and the ultrasonic treatment requires much time, all this leading to a substantial overheating of the melt so that the liquid portion of the solidifying ingot extends beyond the mould.
Another disadvantage of the known method is that structure refinement non-uniformity increases with the size of ingots.
Still another shortcoming of the known method is that the short time the ultrasound acts upon a poorly overheated melt proves to be unsufficiently effective as regards the removal of gaseous and solid nonmetallic impurities.